1. Technical Field
This invention generally relates to optical data processing. In particular, this invention relates to a system and method for converting a temporally short and spectrally broad optical pulse into a train of spectrally narrow and distinct optical pulses.
2. Description of the Related Art
Analog signals are often digitized using an analog-to-digital converter (ADC). Digital signal processing may be performed on the digitized signal. The digital signal processing may require the digitized signal to have been sampled at a high rate and at a high resolution. High resolution means that, for example, many different voltage levels of the analog signal can be distinguished and digitally represented. Conventional electronic ADC may not be able to achieve both high sampling rates and a high resolution. To solve these problems, optical ADCs may instead be used.
The optical ADC may be based on converting a temporally short and spectrally wide optical pulse into a train of spectrally narrow and distinct optical pulses. The temporally short and spectrally wide optical pulse may itself be a part of a sequence of temporally short and spectrally wide optical pulses produced by a laser.
Accordingly, each individual short optical pulse produced by the laser may be transformed into a train of spectrally distinct pulses. The duration of the pulse train may be substantially the same as the time between the start of the individual short optical pulse in the sequence of short optical pulses that was transformed, and the start of a next individual short optical pulse in the sequence of short optical pulses that is to be transformed next.
As an illustrative example, a single optical pulse produced by the laser that is white, (i.e., including many wavelengths) may be transformed into a train of colored pulses (i.e., including a specific wavelength), e.g., red, blue, green, and purple. Each of the colored pulses is modulated with a different time portion of the analog data. Thus, the red pulse may carry a first time portion of the analog data, the blue pulse may carry a second time portion of the analog data, the green pulse may carry a third time portion of the analog data, and the purple pulse may carry a fourth time portion of the analog data. A wavelength demultiplexer may be used to separate the different colored pulses from the train, directing each to a detector and then to a corresponding ADC. Each ADC receives an optical pulse of a particular wavelength (e.g. color). For example, an ADC may process only red pulses. This “red ADC” does not need to convert the blue, green, or purple pulses. Accordingly, the red ADC can operate relatively slowly, and can be a low-bandwidth ADC.
For example, the red ADC may receive a red pulse carrying the first time portion of the analog signal. Next a blue ADC may receive the blue pulse carrying the second time portion. A green ADC may next receive the green pulse, and a purple ADC may next receive the purple pulse. After the purple pulse, another red pulse is received that carries, for example, a fifth time portion of the analog signal. The red ADC needs to process the initial red pulse before it receives another red pulse. Thus, because the red ADC only needs to process the red pulses, which are separated in time by blue, green and purple pulses, the red ADC does not need to operate as fast, even though a high sampling rate may be required. The same applies for the remaining ADCs. Thus, low-bandwidth (i.e., slower) ADCs may be used.
A property of these low-bandwidth ADCs is that they may have a higher resolution than a high bandwidth ADC. This means that a low bandwidth ADC can distinguish among a larger number of amplitude values that a higher bandwidth ADC when converting an analog signal to a digital signal, which solves a shortcoming of conventional electronic ADCs.
As described above, low-bandwidth ADCs may be used instead of high-bandwidth ADCs to increase the available resolution. But in some cases, the bandwidth of an analog signal is so high, that even conventional high-bandwidth ADCs cannot process the analog signal. In this scenario, using multiple ADCs receiving pulses of different wavelengths allow for ADC conversion on these high-bandwidth analog signals, which may not have previously been possible.
FIG. 1 discloses a prior art solution for transforming a white short optical pulse into a multi-colored pulse train for data modulation. FIG. 1 shows a system 100 that includes a mode locked fiber laser (MLFL) 102 outputting a short optical pulse with many wavelengths. Laser 102 sets the polarization of the short optical pulse in a given direction. The short optical pulse travels to polarization beam splitter (PBS) 104. PBS 104 can selectively send an optical pulse in one of two directions, depending on the polarization of the optical pulse. Here, PBS 104 sends the short optical pulse to channel 105 of wavelength division multiplexer (WDM) 106 based on the polarization of the optical pulse set by laser 102. WDM 106 demultiplexes the short optical pulse according to wavelength and outputs a plurality of optical pulses, each associated with a different span of wavelengths (i.e., of a particular color).
The plurality of optical pulses outputted by WDM 106 are combined into a pulse train by passing them back through WDM 106. This is done by connecting each output of WDM 106 to an optical fiber of different length. For example, assume that WDM 106 outputs a red pulse onto channel 108. Channel 108 is attached to a piece of optical fiber that is stretched by fiber stretcher (FS) 110 and delayed by delay loop (DL) 112. The red pulse takes an amount of time to pass through FS 110 and DL 112, and reaches Faraday mirror (FM) 114, attached to the end of the optical fiber. FM 114 flips the polarization of the red pulse and in such a manner that the return path compensates for any polarization distortions in the red pulse caused by travelling through the optical fiber. FM 114 also reflects the red pulse back towards WDM 106.
In addition to channel 108, the other outputs of the WDM are also connected to pieces of optical fiber. Each optical fiber also has a fiber stretcher, a delay loop, and a Faraday mirror. The optical fibers of the different outputs each have delay loops of different lengths, so that the pulses on each of the optical fibers are reflected back to WDM 106 at different times.
WDM 106 thus receives colored reflected pulses in a sequential order. WDM 106 multiplexes these colored reflected pulses into a pulse train. Each pulse in the pulse train has a color (i.e., a wavelength or wavelength range). In this way, a short white optical pulse can be transformed into a pulse train of colored pulses.
The pulse train outputs WDM 106 at channel 105 and reaches PBS 104. Because of the polarization flipping performed by the Faraday mirrors (such as FM 114), the pulse train reaches PBS 104 with a polarization that is opposite to the polarization of the short optical pulse that exited laser 102. Thus, PBS 104 sends the optical pulse train, in a different direction, to erbium doped fiber amplifier 116. Amplifier 116 sends the optical pulse train to modulation system 118, which modulates the optical pulse train with analog data.
A drawback to the prior art system shown in FIG. 1 is that optical fibers are necessary to reflect the colored pulses outputted by WDM 106 back to WDM 106 for multiplexing into the optical pulse train. The additional optical fibers (associated with the fiber stretchers, delay loops, and Faraday mirrors) cause system 100 to be too large. It would be preferable to decrease the size of system 100. In addition, because it is difficult to control the length of the fiber to required degree of accuracy, fiber stretchers are required for additional control, and in some cases, the desired delay accuracy cannot be achieved. In addition, the optical interface between the fiber and the WDM increases optical loss.